Split several PPC instruction classes.

Slight reorganisation of PPC instruction classes for scheduling. No
functionality change for existing subtargets.
 - Clearly separate load/store-with-update instructions from regular loads and stores.
 - Split IntRotateD -> IntRotateD and IntRotateDI
 - Split out fsub and fadd from FPGeneral -> FPAddSub
 - Update existing itineraries

Patch by Tobias von Koch.

llvm-svn: 162729